Well you kind of do you got a head and cam range picked out which will make certain amount of hp in a narrow ish range.
No I don't think it will negatively affect gas mileage a may have a slight increase don't know if it would be noticeable amount though eg 1-2 mpg increase.
My point is say you end up making 300-325hp, right now your comparing ported 920 to stock J's by why not port the J's which will need less cam to make the 300-325hp which to me be more streetable and better on gas since a cam has greater effect on mileage or with same cam you'd have more power with similar mileage.
But since this a temp engine and you already got a decent set of 920 I see nothing wrong with you going that way. Plus I feel mileage is hard to gain but easily lost, so as long you don't do anything to overly kill fuel mileage these builds should come out with similar results give or take a few mpg.
